Color imaging systems are based on the fact that a small number of colors can be mixed in varying proportions to provide a very large range, or gamut of colors. The materials, dyes, inks, phosphors etc. used to provide the different colors of a color imaging system are commonly called colorants. Most common color imaging systems are based on three colorants. An appropriately chosen set of three colorants can provide a large fraction of the all the colors perceived by the visual system. For example, all the colors seen on TV or computer CRT displays are produced by stimulating the eye with light emitted by Red, Green and Blue phosphors. Mixing Cyan, Magenta and Yellow inks provides the colors on most of the printed pages.
However, while three colorants can provide a large gamut of color, imperfections in the colorants used in printing systems limit the gamut achieved with printing systems using three colorants. As a result, color imaging systems using more than three colorants are used in many applications. In particular, for applications requiring very high quality printing, color printing systems using five or even more colorants including a neutral black colorant have been developed and are used. Neutral black in color printing is used to control the saturation and luminance of colors at points in a printed image. White is provided by the white of the paper on which the images are printed. The amount of white at a point in an image is reduced by the amount of black printed at the point. Black is printed as a separate “color” because combinations of the other colorants generally do not provide a sufficiently “black” black. Common printing systems therefore use four colorants. Cyan, Magenta, Yellow plus Black.
In color printing, each of the colorants used to produce color, i.e. each colored ink, is applied to paper as a raster, conventionally called a screen, of very small colored ink dots. The dots vary in size and/or in spacing. In conventional screens, called “angled halftone screens”, the dots are arrayed in a regular pattern of rows with the spacing between dots constant and dot size varying. With the introduction of computers into the printing industry, screens called stochastic screens came into use. In stochastic screens dot spacing is varied “pseudo-randomly” and dot size is either constant or also varied pseudo-randomly. At any point on a printed paper the fraction of the surface of the paper that is covered with the color of a particular ink is proportional to the size or number of the dots of the ink at the point. The relative amounts of each colorant at a point on the paper, and thereby the perceived color of the point, is therefore controlled by the size and/or the number of the dots of each of the colorants at the point.
In conventional printing with angled halftone screens, if the screens of the various colorants are applied so that the rows of dots of different screens are parallel, the slightest relative misalignment of the rows from parallel leads to disturbing patterns in the printed image called moiré patterns. Stochastic screens on the other hand do not have rows of dots or any regular patterns of dots that result in stochastic screens having defined directions. There is no alignment problem with stochastic screens that causes moiré patterns. In stochastic screens moiré patterns are substantially prevented by the randomness of the dot spacing.
In order to reduce or prevent moiré patterns when printing with conventional angled halftone screens, angled halftone screens are printed so that angles (hence, the word “angled” in angled half tone screens) between rows of different angled halftone screens are relatively large. Let the printing angle, hereafter referred to as a “screen angle”, of an angled halftone screen printed on a paper be defined as the angle between 0° and 180° that is measured between the horizontal direction of the printed page and the direction of the rows of dots of the angled halftone screen. In a typical Cyan (C), Magenta (M), Yellow (Y) and Black (K) (hereafter referred to as “CMYK”) printing process, the Cyan, Magenta and Black angled halftone screens have screen angles of 15°, 75° and 45° respectively and the Yellow angled halftone screen has a screen angle of 90°. This results in C and M being printed with screen angles that are separated by 30° from K and from each other, and with Y printed with a screen angle separated by 15° from the screen angles with which C and M are printed. Y is a “weak” color and does not generally generate moiré patterns as readily as do the strong colorants C,M and K. Y can therefore be printed with a screen angle close to the other colors. (In angled half tone screens where the dots are equally spaced from each other, the screens appear to have parallel rows of dots that are 90° to each other, i.e. the screens are invariant under a 90° rotation. Therefore a screen that has rows at 90° also has rows at 0°, a screen that has rows at 15° also has rows at 105°. Therefore the difference between the angles of a screen at 15° and 75° is not 60°, as might be expected, but 30°.)
Most color printing processes using more than four colorants generally use the standard CMYK set of colorants plus additional colorants. In prior art printing using angled halftone screens, each of the additional colorants is printed with a screen angle that is the same as one of the screen angles with which the C, M or Y colorants is printed. Black is generally the only color printed with a screen angle of 45° and it is established practice to print C and M with screen angles that 30° apart from each other and from black, as in the case of printing with the CMYK colorant set, in order to assure minimum moiré interference between these strong colorants. However, it is known and established practice in prior art that two strong colorants that have adjacent hue angles (as defined in typical chromaticity diagrams in which hue is a function of two color variables) should be printed with very different screen angles in order to prevent moiré patterns. A color printing process using C, M, Y and K plus an additional strong colorant having a hue angle intermediate the hue angles of C and M would cause C and M to be printed with the same or very close screen angles. As a result, in prior art angled half tone printing systems using colorant sets comprising N colorants, hereafter referred to as “N-colorant sets”, where N represents the number of colorants used, strong colorants having hue angles between the hue angle of C and M are not used.
For example, a Pantone® Hexachrome™ printing system, which is used with both conventional angled half tone screens and stochastic screens is described in “PANTONE® HEXACHROME™ COLOR SELECTOR”, by Pantone Inc., 1995, which is incorporated herein by reference. This system uses colorants K, C, G, M, Y, and O, none of which has a hue angle intermediate the hue angles of C and M. On the other hand, a 7-colorant set for printing with stochastic screens only is described in a brochure entitled “DAVIS INC HiFi Color Project” published in 1993 by DAVIS INC/Visible Solutions Ltd./Nima Hunter Inc., incorporated herein by reference. This system uses a Violet (V) colorant. Violet has a hue angle intermediate the hue angles of C and M. However, since the DAVIS INC HiFi Color Project system is not intended for use with angled halftone screens, the use of V is not prohibited in this system by prior art practices employed to prevent moiré patterns.
The exclusion of colorants with hue angles intermediate the hue angles of C and M in printing processes using angled halftone screens restricts the gamut of colors available to prior art printing processes using angled halftone screens and more than four colorants. In particular, a large fraction of the colors required in printing applications falls in the color region of chromaticity diagrams lying in the region of the visual gamut between the line connecting C and M and the saturated blue and purple hues. Hues in this color region cannot be produced using prior art N-colorant sets used with angled half tone screens, i.e. the color region is not accessible with these prior art N-colorant sets.
FIG. 1 shows schematically a chromaticity diagram 20 showing the human visual color gamut 22, and portions of visual gamut 22 included in a restricted CMYK gamut 34, and a full CMYK gamut 36, of printed hues provided by a typical CMYK colorant set. Hues in chromaticity diagram 20 are mapped at an arbitrary constant luminance as a function of arbitrary units of commonly used color variables u* and v*. Details of the definition of color variables u* and v* can be found in “THE REPRODUCTION OF COLOUR IN PHOTOGRAPHY, PRINTING AND TELEVISION”, by R. W. G. Hunt; Fountain Press, England; Fourth Edition 1987, which is incorporated herein by reference.
Points in chromaticity diagram 20 on a boundary curve 24 and in the area 26 located inside boundary curve 24 represent the hues of the human visual gamut 22. Curve 24 defines the outer boundary of visual color gamut 22 and comprises a curved part 28 and a straight-line part 30. Any hue perceived by the human visual system is represented by a point on or within boundary curve 24, and any point in area 26 or on boundary curve 24 generally represents a hue perceived by the human visual system.
Fully saturated hues are located on boundary curve 24. Portions of boundary curve 24 corresponding to particular saturated hue sensations are labeled with the names of the particular hue sensations. All points on curved part 28 of boundary curve 24 represent monochromatic light stimuli (i.e. light stimuli comprising a single wavelength of light) and all monochromatic light stimuli are represented by points that fall on curved part 28 of boundary curve 24. Points on curved part 28 of boundary curve 24 representing a number of monochromatic light stimuli are shown and are labeled with the wavelength in nanometers, in parentheses, of the light stimuli they represent. Hues represented by points on straight-line part 30 of boundary curve 24 represent fully saturated purples, magentas and mauves. These hues can only be produced by mixing red and blue light stimuli.
The sensation that a light stimulus is neutral is the sensation that the light stimulus is “hueless”. The hueless, or neutral colors, are the sensations of shades of gray ranging from black to white. The sensation that a light stimulus is neutral is located inside area 26 at a point 32 that is the origin of coordinates of chromaticity diagram 20. Whether point 32 represents white or a shade of gray is a function of the luminance of the light stimulus represented by point 32 (i.e. the arbitrary constant luminance of chromaticity diagram 20) relative to the luminance of a reference white. In printing, the reference white is generally the white of the paper on which colorants are printed. For an area of a printed image, point 32 represents white if no black is printed in the area. As the fraction of the area printed with black colorant increases, point 32 represent an increasingly darker shade of gray.
Since the amount of white at a point in a printed color image is controlled by the amount of Black printed at the point, the colorant K of the CMYK colorant set (and of all colorant sets using a Black colorant) is located at point 32. Point 32 is labeled W/K to indicate that it represents shades of gray produced by printing black on the white provided by the paper on which the image is printed.
Points labeled C, M and Y in chromaticity diagram 20 represent the hues of colorants C, M and Y respectively. Points labeled (C+Y), (M+Y) and (C+M) represent hues produced by mixing equal amounts of the colorants appearing in the respective parentheses. Restricted gamut 34 is defined by the triangle formed by short dash lines 38 that connect points C, M and Y. Full gamut 36 is defined by the polygon formed by long dash lines 40 connecting points C→(C+M)→M→(M+Y)→Y→(Y+C)→C.
The C, M and Y colorants are subtractive colorants. Each absorbs light having a wavelength in a different restricted band, an “absorption band”, of the visual spectrum and transmits light having a wavelength in the rest of the visible spectrum. If the C, M and Y colorants were ideal subtractive colorants, the points (C+M), (M+Y) and (Y+C) would be on the lines connecting points C and M, M and Y, and Y and C, respectively. Restricted and fall CMYK gamuts 34 and 36 respectively would then be congruent. To the extent that the C, M and Y colorants are not ideal subtractive colorants, but are real world colorants, they transmit light where they should not do so and absorb some of the incident light that should be transmitted completely. This causes restricted gamut 34 to shrink. It also results in mixtures of any two of the C, M and Y colorants to be slightly more saturated than would be the case if the colorants were ideal. As a result, mixing equal parts of C and Y does not produce a green hue represented by a point on the line connecting C and Y in chromaticity diagram 20. Mixing equal parts of C and Y produces a more saturated green hue represented by point (C+Y) that is displaced from the line connecting C and Y, in a direction towards boundary curve 24 of visual gamut 22. Similarly, hues (M+Y) and (C+M) are not located on the lines connecting M and Y, and C and M, respectively, but are displaced towards boundary curve 24 of visual gamut 22.
Full CMYK gamut 36 represents the full range of hues that can be produced by the C, M, Y and K colorants. The different hues within full CMYK gamut 36 can be produced at a point in a printed image by printing different relative amounts of C, Y, M and K at the point. Full CMYK gamut 36 cannot be extended to cover a significant color region in the corner of visual gamut 22 near to the saturated blue and purple hues without the use of a colorant having a hue angle intermediate the hue angles of C and M. (Such a colorant would be represented by a point far outside of extended gamut 36 that is located on a line from point 32 that lies between the lines from point 32 to C and M.) Such colorants are excluded in prior art N-colorant sets for printing with angled half tone screens in order to prevent moiré patterns.
In order to expand the gamut of colors available to printing processes using angled half tone screens and more than the basic C, M, Y and K colorants, it would be advantageous to be able to use strong colorants with hue angles intermediate the hue angles of Cyan and Magenta.